The present invention relates to an optical non-resonating laser diode amplifier, comprising an active material strip forming an amplifying optical guide, covered with a confinement sheath and defined by anti-reflection coating layers.
The invention is particularly suitable for use in linear repeaters or receiving pre-amplifiers in transmission systems incorporating optical fibers which do not preserve light polarization. That situation is found in most present-day communication systems.
For obtaining a non-resonant laser diode optical amplifier, the reflectivities of its end faces should be reduced, with dielectric antireflection coatings, for giving a low value, typically less than 0.1 to: ##EQU1## where G is the internal gain of the laser diode and R1, R2 designate the residual reflectivities of the faces.
This condition would be well defined if a single value had to be taken into consideration for G, R1 and R2. It is unfortunately not fulfilled for:
due to its very structure, the laser diode amplifier has two inherent polarization states which correspond to different internal gains G: in a currently available laser diode operating at a wavelength of 1.5 .mu.m, whose active material strip has a thickness d between 0.1 and 0.2 micron, a width between 1 and 2 microns and a length L of about 500 microns, the geometric dissymetry gives to the optical guide formed by the laser diode two inherent non-degenerated polarized states, having different propagation constants, consequently different confinement factors and a difference between the gains all the higher as the thickness d of the strip is smaller.
the residual modal reflectivity of the anti-reflection coatings depends on the polarization, for the reflectivity minima for two mutually orthogonal polarizations are situated at different wavelengths.
It can be seen that the overall gain of the amplifier has a sensitivity to the polarization of the signal with two separate origins.
It is known that the difference between internal gains corresponding to the TE and TM modes can be reduced by increasing the thickness d of the guide. But any thickness increase moves apart the wavelengths corresponding to the reflectivity minima for the TE and TM polarizations, so the difference between the modal reflectivities for the wavelength of the laser diode.